Diesel-type engines are well known for being highly durable and fuel efficient. Because of this durability and fuel efficiency, diesel-type engines have long been used in heavy-duty motor vehicles, such as trucks, buses and locomotives.
Diesel fuel can be made from any number of components that are produced in an oil refinery. The American Society for Testing and Materials (ASTM) has set specifications for diesel fuels. Some states, including California, have adopted those specifications as legal requirements.
Diesel fuels may be produced from refinery streams that have the qualities required to meet the ASTM specifications. No. 2 diesel fuel is produced from hydrocarbon stocks that are referred to as gas oils. No. 1 diesel fuel is produced from refinery streams that lie in the kerosene boiling range. All fuels that lie within the boiling range of No. 1 and No. 2 diesel fuel are referred to generically as "distillate".
Historically, diesel engines have been operated on a petroleum-derived liquid hydrocarbon fuel boiling in the range of about 300.degree. F.-750.degree. F. (149.degree. C.-399.degree. C.). Modern petroleum refineries can produce high quality diesel fuels containing large straight-chain paraffins. However, due to competing demands for other products, limitations imposed by poor quality heavy crude oils, and other factors, refineries frequently are unable to meet the total demand for such diesel fuels. Many believe that projected demand for diesel fuels will increase by 26 percent from the year 1990 to the year 2010.
Diesel engines rely on compression ignition of the fuel. The source of ignition energy is the high temperature, high pressure air present in the combustion chamber towards the end of the compression stroke. To achieve these high temperatures and pressures, diesel engines typically have far higher compression ratios than do gasoline engines.
Upon injection into the combustion chamber, fuel must quickly mix with air to form a flammable mixture and the mixture must ignite. Since there is normally no additional means for ignition (such as the spark plug in gasoline engines), the fuel must self-ignite, a process called "autoignition". This process takes time and is influenced greatly by the engine combustion system and by fuel properties.
Diesel engines use the heat developed by compressing a charge of air to ignite the fuel injected into the engine cylinder after the air is compressed. More specifically, in the engine, the air is first compressed, then fuel is injected into the cylinder; as fuel contacts the heated air, it vaporizes and finally begins to burn as the self-ignition temperature is reached. Additional fuel is injected during the compression stroke and this fuel burns almost instantaneously, once the initial flame has been established.
A period of time elapses between the beginning of fuel injection and the appearance of a flame in the cylinder. This period is commonly called "ignition delay" and is a major factor in regard to the performance of a diesel fuel. If ignition delay is too long, the fuel will accumulate in the cylinder until it reached ignition conditions and then will burn rapidly, causing a sudden pressure increase which may result in engine knocking. Too long an ignition delay may result in a smokey exhaust, a decrease in engine efficiency, and possibly dilution of the crank case oil. A reduction in ignition delay can be obtained by varying the chemical nature of the injected fuel. Straight-chain paraffinic hydrocarbons give the least ignition delay, while branched-chain paraffins and cyclic (including aromatic) hydrocarbons tend to have poorer ignition characteristics. Aromatic hydrocarbons are relatively compact and unreactive molecules compared to the other prime constituents of diesel fuel. Consequently, aromatics tend to resist ignition and be low in cetane number.
For this reason, n-hexadecane ("cetane") has long been used as a standard reference material for determining the ignition quality of commercial diesel fuels. A scale called "cetane number" has been devised for ranking the relative ignition delay characteristics of a given diesel fuel. The cetane number of an unknown fuel is determined by comparing its ignition delay in a standard test engine with reference fuels which are prepared by blending cetane (assigned a rating of 100) and 2,2,4,4,6,8,8 heptamethylnonane (assigned a rating of 15) until a reference fuel is found to have the same ignition delay characteristics as the unknown fuel; the cetane number is obtained by the equation: EQU Cetane No.=(Vol. % Cetane)+[0.15 (Vol. % Heptamethylnonane)]
In general, large stationary engines which run at fairly constant speeds and loads have the lowest cetane number requirements (e.g., 30 to 45), while smaller motor vehicle diesel engines have the highest requirements (e.g., 40 to for obtaining optimum performance.
One of the more important difficulties that arises through the use of diesel engines is the problem of starting the engine when it is cold. Fuels with high cetane numbers have the advantage of giving relatively easy starting at low temperatures. Additionally, fuels with high cetane numbers reduce destructive combustion knock, provide more efficient combustion and smooth engine operation, lower maximum cylinder pressures, and reduce carbon deposits on cylinder heads.
Cetane rating, which is measured (and defined) by a standard cetane test engine according to a procedure carefully detailed in American Society for Testing and Materials Standard D-613, Test Method for Ignition Quality of Diesel Fuels by the Cetane Method, is critical. A high cetane rating means the fuel autoignites relatively easily and exhibits a shorter ignition delay than a fuel with a low cetane rating, the reverse of the octane rating of gasoline where octane measures a fuel's resistance to autoignition.
Through the years, many types of additives have been prepared to raise the cetane number of diesel fuels. Such additives usually contain nitrogen or sulfur, both of which are known cetane improvers under certain circumstances. The most popular additives, for example, appear to be hexyl or octyl nitrate. However, these additives are highly combustible, as are most of the organic nitrogen- or sulfur-additives commonly used. Further, the nitrogen-containing compounds can add to an engine's NO.sub.x emissions, which contribute to photochemical reactions known to cause smog formation, as well as formation of nitric acid, a factor in acid rain. The sulfur-containing compounds contribute to SO.sub.x formation which can contaminate the lube oil by forming sulfuric acid which breaks down various antiwear additives found in the oil. Also, SO.sub.x emissions contribute to the formation of particulate matter in the form of sulfates which must be emitted from the exhaust.
Despite many years of work and billions of dollars spent by both government and industry in fighting air pollution, air quality problems remain a major problem. Motor vehicles powered by diesel combustion engines are a contributing factor to many of these problems.
Research over the past few years has focused attention on the fact that increasing the cetane number of typical diesel fuels has beneficial effects on diesel exhaust emissions. However, diesel combustion is a very complex process far more so than gasoline engine combustion. Much is yet to be learned about the detailed mechanisms leading to emissions from diesel engines. As just one example, it is known that various diesel engine designs respond very differently to changes in fuel characteristics. Some engines will exhibit dramatic effects in one direction, others in the opposite direction, and still others will not respond at all. This is because engine hardware comprising the injection system, the combustion chamber design, the inlet system design, among many other parts of the engine, affect engine performance and emissions far more than do variations in the normal range of fuel characteristics.
In the production of diesel fuels in a refinery, some process modifications are available, but at an extremely high cost. Process options required to meet new diesel fuel standards have been projected to cost over $2 billion in California alone. The non-process options include (I) the segregation of the regulated vehicular No. 2 diesel fuel from other distillate products so that only the regulated No. 2 diesel fuel would be required to meet regulations regarding low aromatic hydrocarbon/low sulfur content standards, and (II) the purchase and importation of low aromatic hydrocarbon/low sulfur blendstocks from outside California.
The potential process options referred to above include the olefins-to-gasoline process. Also of potential for process options are certain distillate processes, which employs methanol as a feedstock, and several hydroprocessing options which use hydrogen and a catalyst to reduce the aromatic hydrocarbon content.
Higher concentrations of fuel aromatics, beyond their effect on cetane number, also appear to cause small increases in particulate and oxides of nitrogen emissions. In general, aromatic hydrocarbons have a higher flame temperature relative to the other prominent components of diesel fuel. This higher temperature may, by itself, lead to higher oxides of nitrogen formation.
The State of California has defined a diesel fuel containing an extremely low level of aromatic compounds for adoption by fuel suppliers for sale in the State, hereinafter referred to as "California LAD". While California LAD may have advantages in the form of lower exhaust emissions relative to diesel fuel produced and sold today, the production cost impact upon refiners who must adopt processes to produce California LAD is enormous.
The proposed California regulation requires a minimum of 10% aromatics content in vehicular diesel fuel starting on Oct. 1, 1993. Compliance with the low aromatics rule could require major investments in California refineries. Refiners have the potentially less costly option of producing a higher aromatics diesel fuel if they can demonstrate equivalent emissions relative to a 10% aromatics reference fuel.
Lowering the aromatics content of diesel fuel from the current levels of well over 30% to those below 10% requires major capital investment and operating cost for severe hydrotreating processes in most California refineries. This is a severe financial burden during a period in which very large capital funds are needed to make the many changes required for producing reformulated gasoline and for complying with a range of other environmental regulations.
The California Air Resources Board has allowed fuel producers the option of producing a less costly alternative fuel with a higher aromatics content, if equivalent emissions can be demonstrated. We took on the challenge of developing such an alternative fuel and were successful in receiving the first certification from the California Air Resources Board for an alternative diesel fuel. Thus, while our invention did not produce a diesel fuel with lower than 10% aromatics, we were successful in producing a higher aromatics diesel fuel which demonstrated an equivalent emissions relative to the 10% aromatics reference fuel.
A candidate fuel to be tested for emissions equivalency must meet the ASTM D975 diesel fuel specifications. In addition, the following five fuel properties must also be determined:
1. Sulfur content (not to exceed 500 ppm); PA1 2. Total aromatic hydrocarbon content; PA1 3. Polycyclic aromatic hydrocarbon content; PA1 4. Nitrogen content; PA1 5. Cetane number.
Once the fuel is certified "equivalent" a producer can market the equivalent fuel as long as the first four of the above properties are not exceeded. The above determined cetane number is the minimum allowable.
The goal we achieved was to obtain a fuel which would pass the certification test, yet be as economical as possible to produce. While fuels with higher cetane numbers and lower aromatics relative to the fuels of this invention may also be demonstrated to be better than or equivalent relative to a 10% aromatics reference fuel, as recommended by the California Air Resources Board, such fuels would be more expensive to produce; i.e., raising the cetane number and/or lowering the aromatics content involves either extra processing steps or extra additives or both which increases the cost of the fuel.
Thus, we have found a balance between cetane number and aromatics content which provides an economical fuel which passes the California Air Resources Board certification test.
It is clear that formulation of a diesel fuel which minimizes the cost impact on fuel refiners and fully meets the emissions requirements of the California Air Resources Board is much desired.